Control systems of the closed-loop or servo loop type are known wherein the control operates an output device so that a desired or commanded load condition is attained. One example of such a control is disclosed in Burandt et al U.S. Pat. No. 4,487,109, assigned to the assignee of the instant application and the disclosure of which is hereby incorporated by reference. The control disclosed in this patent is particularly adapted for use in operating a power drive unit which in turn drives a load, for example a component of a flight control system of an aircraft. The power unit includes a swashplate or wobbler, the displacement of which is adjustable by means of a servo valve and a control cylinder to vary the torque developed at an output shaft of the power unit. The control system includes transducers for sensing the speed of the output shaft, the displacement of the wobbler and the position of the load. These transducers are coupled to first inputs of a series of summing junctions which develop error signals representing the deviation of an actual output condition from a desired condition. More specifically, a first summing junction receives a command signal at a second input thereof. The output of the first summing junction is coupled to the second input of the second summing junction, and the output of the second summing junction is coupled to the second input of the third summing junction. The output of the third summing junction is coupled to a current driver to signal inputs of the servo valve.
In operation, the wobbler displacement is controlled in accordance with the command signal so that the load is driven to a desired or commanded position.
The wobbler displacement is variable between mechanical stops or limits. At times, it may occur that the command signal is such that the wobbler is driven into contact with one of the mechanical stops. This could conceivably cause damage to the wobbler and/or to other components in the power unit. This condition, termed "mechanical saturation", should be avoided, if possible.
Mechanical saturation can be avoided by introducing electrical saturation in the control system. This may be accomplished through the use of a limiter which limits the input command signal to values which insure that the wobbler will not be driven into contact with one of the stops. Most types of limiters are static in nature, i.e. the positive and negative saturation points for the limiter are determined in advance and are not varied during operation of the system. While such limiters prevent the undesirable mechanical saturation noted above, they tend to unduly limit the magnitude of the error signals generated by the summing junctions. This in turn introduces an undesirable increase in response time of the wobbler, which may in turn render the control system unsuitable in certain applications requiring fast response times.
Dynamic limiters have been devised wherein the positive and negative saturation points of the limiter are dynamically controlled. This is accomplished by utilizing a dynamically variable voltage source as a control input to the limiter. Such a limiter is disclosed in Beaudette U.S. Pat. No. 3,999,084. However, Beaudette does not disclose the precise construction of the variable voltage source, other than to note that it may comprise, for example, an amplifier. There is not disclosure or suggestion as to the nature of the signal developed by the variable voltage source, nor is there any disclosure or suggestion of how and in what type of circuit such a limiter might be used.
Furthermore, the Beudette limiter requires the use of multiple operational amplifiers, and hence it is a relatively complex circuit.